The present invention relates to the field of well tools located downhole in a borehole. More particularly, the invention relates to an apparatus and method for locating and for retrieving downhole well tools.
Logging tools and other devices are run downhole in boreholes to investigate subterranean structures and to perform different exploratory and production operations. Logging tools are typically lowered into a borehole with a wireline which provides a structural connection to surface equipment and further provides a conductor for transmitting power and electrical signals. Wirelines are subject to failure due to wear and to binding forces exerted by the borehole wall against the tool. When a wireline separates from a logging tool in a vertical or inclined borehole, gravity pulls the tool lower within the borehole. Accordingly, the last known location of the logging tool may not accurately reflect the final resting location of the tool. Such location may be particularly difficult to locate in multilateral wells having multiple borehole branches.
Rig down time caused by a lost logging tool is costly. Wireline failure can strand a logging tool thousands of feet downhole in borehole, and abandonment of a million dollar logging tool is not economic. Loose logging tools impede further rig operations, and drilling rig time on offshore wells can cost over one hundred thousand dollars per day. This cost is increased by the travel time required by well contractors specializing in tool retrieval. When a tool is lost in a well, such well contractors may travel for one or more days to reach the borehole site before the tool retrieval operations can begin. Although certain of these costs can be avoided by effective equipment maintenance, logging tools are also lost due to tool sticking, formation collapse, and other causes unrelated to the wireline.
The process of locating and retrieving a lost tool (known as a "fish") is known as "fishing". Casing collar search tools are sometimes successful in locating lost tools. Alternatively, fishing operations are typically conducted by lowering a drill string into the borehole until the drill string lower end contacts the lost tool. Such contact places weight on the lost tool and reduces the weight on the drill string. If the tool is lodged in the borehole, the drill string weight can further drive the tool into the geologic formations. The reduction in drill string weight is monitored to identify contact with the downhole tool, however the weight reduction is almost imperceptible in deep boreholes requiring a long drill string. Additionally, false weight readings can occur in deviated and horizontal wells as the drill string contacts the borehole wall.
After the lower end of the drill string has located the downhole tool, an "overshot" is attached to the tool for retrieval to the borehole surface. Overshots typically comprise a coiled steel ribbon which is lowered over a tool. The overshot constricts to grip the tool as the drill string and overshot are withdrawn from the borehole. Overshots are effective when the tool is stuck in the borehole and the wireline is still attached to the tool. In such circumstances, the wireline guides the overshot over the tool end so that an effective grip can be achieved. However, overshots are difficult to operate when the tool has parted from the wireline and the tool location is unknown.
Accordingly, a need exists for an improved system for locating and retrieving tools downhole in a borehole. The system should be easy to deploy and to operate, and should be sufficiently flexible to handle different lost tool conditions.